The compass is the instrument that lets us find the direction of the magnetic North Pole at a glance.
A compass usually consists of a magnetised needle, or a thin magnetised metal strip shaped like an arrow, balanced on a small pointed pivot.
The metal strip can rotate through 360° inside a graduated dial, naturally lining itself up along the North-South axis.
What's the best compass for hiking?
There are various types of compass, each designed for specific uses: the baseplate compass, the prismatic compass, the steering compass, the electronic compass, the aircraft compass, the manual sighting compass.
The most widespread among hikers, because it is so practical to use together with a map, is the so-called “baseplate compass”, also known as the “protractor compass”.
This type of compass consists of a plate of transparent plastic on which a rotating circular dial graduated to 360° is mounted, with the cardinal points marked on its edge. This dial also carries a series of parallel lines drawn inside it, called “meridians”, running parallel to the axis set by the north and south points marked on its edge, and a thicker line running from the centre of the dial to the north point on the edge, called the “orienting arrow”.
A rotating magnetised needle is mounted in the centre of the dial, while an arrow called the “direction-of-travel arrow” is drawn on the transparent plate.
To use the compass accurately together with maps, you need to remember that the geographic North Pole (the one on maps, marked by the point where the earth's meridians meet) does not coincide with the magnetic North Pole and that, while the former is a fixed point, the latter varies from place to place and over time. The difference between the positions of these two Poles can be measured by the angle formed by the direction of magnetic north and that of geographic north, called “magnetic declination”.
Depending on where we are, magnetic declination can be positive (east) or negative (west). To make it clearer, let's say it is positive if, when we point the orienting arrow drawn on the dial of our compass towards geographic north, we see the compass's magnetised needle swing further towards the east. In this case, then, magnetic north lies east of the meridian we are on.
In Italy magnetic declination is around 1-2° and decreases fairly steadily by about 5' (five minutes of arc) a year.
It will be enough to subtract 1° from the value of the declination angle to know the true position of the magnetic north pole. Maps from the Istituto Geografico Militare show the magnetic declination and the date it was measured. If the map we are using is not up to date enough, it will be enough to subtract 5' a year to get a rough estimate of the new magnetic declination at its current value.
To use a compass correctly you need to carry out two operations:
1- Setting the compass.
Once the magnetised arrow has settled in the direction of magnetic North, slowly turn the whole compass until the orienting arrow coincides with the magnetised needle.
By doing this we can say we have set the compass, which lets us locate the cardinal points.
2- Finding the direction.
On the map we lay the transparent plate of the compass (or rather: we position the compass so that the direction-of-travel arrow drawn on the transparent base points) so that the direction-of-travel arrow points towards the destination we want to reach. The angle formed between the direction-of-travel arrow and the orienting arrow (North) is called the azimuth.
By turning the rotating dial we have thus also fixed our direction of travel, which lets us quickly check which way to go simply by finding the position of magnetic North.
Precautions when using a compass
Warning: compasses are extremely sensitive instruments and can have trouble finding magnetic North because of magnetised objects the hiker is unknowingly wearing.
These magnetised objects can shift the position of the magnetic needle, leading the hiker to hold a wrong azimuth and therefore a wrong direction.
To avoid this problem all you need to do is test the compass in an area where you know the directions of the cardinal points, wearing everything you expect to carry with you (including metal flasks, electronic altimeters, watches, mobile phones and GPS devices).
This way you can check for yourself whether the compass needle is affected by any of the objects you are carrying.
Before using a compass, brush up on the basics in our introduction to navigation in the mountains.
Frequently asked questions
What's the best compass for hiking?
The most widespread among hikers is the baseplate compass, also called the protractor compass: a transparent plate with a rotating dial graduated to 360 degrees, a magnetised needle and a direction-of-travel arrow. It is practical because it is easy to use together with a map.
How do you set a compass?
Once the magnetised needle has settled towards magnetic North, slowly turn the whole compass until the orienting arrow lines up with the needle. At that point the compass is set and lets you find the cardinal points.